1. Field of the Invention
This invention relates to the field of printed circuit assemblies manufacturing and, in particular, to processes for manufacturing printed circuit boards.
2. Prior Art
Since their discovery, printed circuit techniques have become a commercially important part of almost every area of the electronics industry. Printed circuitry has virtually become the only economically viable technique for the interconnection of components in even fairly low volume production. In high volume production, printed circuitry is universally employed. Recent technological developments in integrated circuits and their consequent widespread acceptance and use in essentially every phase of consumer, commercial, industrial and military and aerospace electronics have resulted in two conflicting demands being placed upon printed circuit manufacturers. First, the complexity of the interconnection required, coupled with the inherent miniaturization of integrated circuits themselves, have led to increased demands for further and further miniaturization of printed circuits in order to accommodate the desired integrated circuit functions within a package which does not become so large that the advantage of miniaturization is lost in the interconnection process. Second, as circuit densities increase in response to the demand for miniaturization and the complexity and number of interconnection likewise increase, the opportunities for failure similarly increase thereby giving rise to a demand for inherently higher reliability interconnection techniques. The response to these demands has been a series of improvements in printed circuit manufacturing processes with consequently higher densities and reliabilities being obtained. Still further improvement is needed, however, in order to allow further miniaturization and reliability improvements. With the advent of further minitiaturization, the integration of surface mount components into printed circuit boards has become important. Of particular concern with surface mounted components is the cost and accuracy of applying additional solderable material to the surface mount component connection points.
The superiority of the present invention in overcoming these problems may best be seen in its relationship to the three commonly used processes which make up the bulk of the present day volume in surface mounting techniques. Present techniques call for applying a mixture of flux and very small balls of solder to each location where a surface mounted component lead will be attached to the substrate. The solder paste is very costly ($22.00/oz.) and difficult to apply. Application techniques now in use include silk screening the entire pattern on one side of the part. A second process involves dabbing or squirting the solder paste onto each location. The components are then placed on the solder paste "blobs" and are normally held in place by the surface tension of the solder paste.
The assembly is then raised to soldering temperature through the use of ovens, infrared heat, or "vapor phase systems" (such as that manufactured by HTC).
Another technique is to glue the components in place by applying an adhesive to the substrate in the area underneath the component, placing the component and allowing the adhesive to "set." The assembly is then passed over a "wave" solder, component side down, and the connections are soldered. This process may be used with single sided, double sided, or "multilayer" substrates.
A typical plated-through hole printed circuit assembly using the conventional and well known copper-tin/lead technique is accomplished as follows:
A substrate of copper-clad insulating material, usually fiberglass-epoxy, is drilled to a pre-established hole pattern suitable for the accommodation of component leads to be mounted on the board in accordance with requirements established during the preparation of the art-work layout. On the drilled board, a thin layer of copper is deposited by electroless chemical deposition. The purpose of this thin layer is to provide a continuous conducting path over the entire surface of the board, including the walls of the previously drilled holes. A negative image of plating resist is then applied to the thinly-plated board by either silk-screening or by a dry-film process, leaving exposed only those portions of the boards on which it is desired to have circuit conductors.
After selectively masking the board as described above, the exposed areas of previously deposited copper, including the hole walls, are electroplated with copper to build up the circuit traces to the desired thickness. A plating of tin/lead is then applied over the exposed copper in the desired thickness to provide a solder compatible surface for component attachment.
The negative image resist is next chemically stripped leaving exposed the remaining thin layer of chemically deposited copper which is then etched away by immersion in the etching bath.
Heat is then applied to the printed circuitry to cause the tin/lead plating to amalgamate into solder. If desired, a solder mask may be applied over the board by silk-screen techniques, leaving only those portions of the circuit to which connection of component leads or discrete wiring, or on which connections will be installed, uncovered. By using solder mask in this and other processes, "bridging" of closely spaced circuit paths is avoided when components are mounted and the printed circuitry is soldered in a "wave" soldering process, or, if surface mounted components are required, the solder paste is applied and the components mounted and soldered as described previously.
See for example U.S. Pat. No. 4,104,111.
It is an object of the present invention to improve the reliability, and at the same time reduce the overall manufacturing cost of materials and labor, of surface mounted component assemblies while providing the necessary and sufficient solder for the surface mounted components. Cost savings are realized through the use of chemical plated material from a large anode as opposed to the solder paste.
It is another object of the present invention to apply the material necessary to make an adequate solder joint at exactly the proper locations in a manner much more precisely controlled than the present silk-screen or drop dispersement method.